The Rankine cycle is the fundamental operating cycle of all power plants where an operating fluid is continuously evaporated and condensed. A closed Rankine cycle system includes a boiler or evaporator for the evaporation of an operating fluid, a turbine (or other expander) fed with the vapor to drive a generator or other load, a condenser for condensing the exhaust vapors from the turbine back to liquid, and a pump for recycling the condensed fluid to the boiler/evaporator. Operating fluids for Rankine cycle systems include water and organic refrigerants such as R-245fa or R134a. Selection of operating fluid depends mainly on the temperature range at which the Rankine cycle system will operate, with organic refrigerants best suited to lower operating temperatures and water/steam being best suited for higher operating temperatures. Low operating temperatures may prevail in a waste heat recovery application, while low operating temperatures may be desirable in some small-scale systems configured for use in residential or small business structures. High operating temperatures can result in greater efficiency, but present issues of heat containment and recovery. The division between low operating temperatures and high operating temperatures is generally about 300° F. (148° C.)-350° F. (177° C.).
Steam is used for a wide variety of processes and is commonly employed as an operating fluid in Rankine cycle systems to convert thermal energy into mechanical work, which can be used to generate electricity. The most common way of generating steam is to combust fuel to release heat, which is transferred to water in a heat exchanger which may be referred to as a boiler. Steam boilers typically separate the water into channels or tubes to expand the surface area and enhance transfer of heat to the water. Many steam boilers employ arrangements to recover heat from the exhaust gasses after the gasses have been used to generate steam. Boilers commonly employ housings and insulation to contain the heat from combustion and focus the heat on tubes containing the water. Different arrangements of steam tubes are employed to enhance heat transfer from the hot combustion gasses to the water.
Steam can be generated for delivery at temperatures ranging from 212° F. (100° C.) to temperatures above 900° F. (500° C.). Steam may form at temperatures below 212° F. (100° C.) in low pressure environments, but may have limited utility. Low temperature “saturated” steam is preferred for heating applications, while high temperature “superheated” steam is preferred for power generation and turbines. Superheated steam is steam at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured. It will be apparent that generating superheated steam at temperatures above 350° F. requires a higher intensity of heat than generating low temperature saturated steam. The concentrated heat necessary to generate superheated steam for use in a turbine creates challenges in terms of heat containment and recovery when compared to lower temperature systems. For example, exhaust gasses leaving a combustion chamber where superheated steam is generated will be at least as hot as the steam, meaning that significant energy must be recovered from the exhaust gasses to maintain efficiency of the system. Heat lost by conduction and radiation can damage sensitive system components and surrounding materials, and represents potential system inefficiency.
In systems that employ steam to generate electricity, superheated steam is delivered to an expander such as a steam turbine. As the steam passes through the turbine, it delivers motive force to turn a generator, and leaves the turbine as steam at a lower temperature and pressure. After passing through the expander, steam is cooled and condensed back to liquid water in a heat exchanger dedicated to this purpose called a “condenser.” This liquid water is then pumped back into the steam generator to complete the cycle. The condenser may be configured to deliver the heat recovered from the turbine exhaust to another system, such as domestic hot water, hydronic heating systems, or an evaporative cooling system such as an absorption chiller. Heat is also commonly recovered from the exhaust gasses leaving the steam generator.
It is common for combined heat and power systems to employ three heat exchange assemblies: the heat source/steam tube exchanger; the condenser; and an exhaust gas heat recovery heat exchanger. These three heat exchangers are typically provided as separate assemblies, which occupy significant space, is inefficient in terms of manufacturing cost, increases the number of potential points of failure, and allows heat leakage by radiation and conduction to the surrounding environment. Large scale steam driven electric generators are typically situated in dedicated purpose-built structures, and are operated by trained personnel. Small scale micro CHP equipment designed for installation in the mechanical room of a home or a small business must be extremely compact and release small amounts of heat to the surrounding environment.
Small scale or “micro” combined heat and power (CHP) systems are being developed for use in residential structures and small businesses. These systems generate steam and employ a steam turbine to generate electricity, with heat recovered from exhaust gasses and the condenser for use by the home or business owner. Micro CHP systems provide back-up power generation, low cost electricity, and heat in a single system, making them attractive alternatives to conventional heating systems. Further, micro CHP systems can be connected to communicate with each other and provide coordinated response to peak power demand or load absorption when renewable sources place excess power on the grid.
There is a need for a compact and cost effective arrangement of a steam generator, turbine, and heat exchangers suitable for micro CHP systems to be installed in residential and small business structures.
There is a need for a compact and thermally efficient arrangement of heat exchangers for use in micro CHP systems which limit heat released to the surrounding environment.